This invention pertains to the use of nylon in packaging, and especially where additives are to be incorporated into the nylon composition.
Typically, polymer compositions are supplied to fabricators as pellets. It is most convenient that any material which is to be incorporated into the polymer composition in the fabrication of a shaped product with the polymer also be in the form of pellets. The fabricator can then mix the appropriate quantity of the additive pellets with the polymer pellets to obtain the desired composition of polymer plus additive.
Additives, in general, are not obtained from their manufacturers in pellet form. For example, slip agents are typically oily in nature and are not susceptible to being fabricated directly into pellets. Antiblock agents are solid in nature, and their fabrication into pellets the size of the polymer pellets delivered by the polymer manufacturer would prevent their proper dispersion in the overall polymer composition when an article is fabricated from the polymer. It is critical that solid particles be very small in size as compared to the typical size of pellets delivered by the polymer manufacturer. Such polymer pellets are of the order of 3 mm diameter by 3-10 mm in length, for example.
In some cases, additives can be injected directly into an extruder during the extrusion process of fabricating a film. This is one way in which liquid slip agents are incorporated into extrusion operations.
Another common method of incorporating additives into a polymer composition at the time that a polymer is fabricated into a specific article is to first form a concentrate with the additive. Forming the concentrate consists of mixing the additive material into a polymeric carrier in an amount substantially greater than the amount anticipated to be used in the final product. In general, the greatest economy of process is obtained where the additive material is incorporated into the concentrate in as great an amount as possible. For example, some additives can be incorporated into the concentrate in an amount of about 20% additive and 80% polymer. Others can be incorporated in amounts as high as 40% to 50%. Still others can be incorporated in amounts as high as up to about 80% additive, 20% polymer.
Each combination of additive and carrier polymer will have its own particular limitations as far as the maximum amount of additive which can be incorporated into that concentrate. The limitations may be, for example, the amount of the additive which may bleed out of the concentrate if the additive is generally of liquid nature. The limitation may, rather, be related to the abrasiveness of solid particulate additive materials such as silica, where the limitation is more one of the ability of the polymer to serve as a lubricant to prevent the abrasiveness of the solid silica material from severely eroding the equipment used in mixing, or otherwise forming the concentrate composition.
Certain types of additives such as pigments and fillers are incorporated into the final product, such as molded articles, in such large amounts that the additive comprises at least 20% to 50%, and sometimes up to 80% by weight of the composition of the final product. In such cases, where the additive comprises a relatively large fraction of the overall composition, it is well accepted that the carrier polymer for additive concentrates makes a significant contribution to the physical properties of the final product. So, the concentrate carrier polymer is carefully selected, as a polymer which supports the desired polymer properties. Typically, the selected concentrate carrier polymer is readily mixable with the base polymer to make a uniform blend. Thus, it is common for the concentrate polymer to be selected from the same family as the base polymer. Within this context, for example, a pigment or filler material to be incorporated into a polypropylene base polymer composition in an amount of 30% by weight is typically compounded into a concentrate using a polypropylene or a propylene copolymer as a carrier polymer.
On the other hand, where small amounts, namely less than 10%, of an additive material are to be incorporated into a base polymer, conventional practice has been to generally disregard the physical properties of the additive concentrate carrier polymer, and to assume that especially the physical properties of the final product will be controlled by the properties of the base polymer. In that light, the selection of an additive concentrate is based heavily on the additive content, with little significance being attributed to the carrier polymer. As such concentrates are typically used in amounts of less than 5% by weight, most commonly about 2%-3%, conventional knowledge holds that the composition of the carrier polymer can safely be ignored. Thus, the readily available concentrates using olefinic carriers such as ethylene and its copolymers are conventionally used, and are routinely specified for use, in small amounts in such engineering resins as nylon, polyester, and the like.
Additive concentrates are available commercially, and are used in substantial quantities in modifying the properties of a wide variety of polymer compositions. The inventors herein have found that these commercially available concentrates work relatively well with certain polymers, and especially chemically related olefin polymers and copolymers; but that the commercially available additives tend to cause certain reductions in desirable properties when incorporated into nylon composition, even in small amounts. For example, the haze level is increased and the tensile strength tends to be decreased, with addition of only 2%-3% conventional additive concentrate. Certain of the other strength properties are likewise somewhat degraded by the incorporation of the concentrate additive. However, in some cases, it is highly desirable to include the additive composition in the nylon to obtain certain desirable properties. In those applications, the instant inventors have found that the incorporation of even small amounts of the additives to obtain the benefits thereof includes the undesirable side effect of reduction in other desirable properties.
It is an object of this invention to provide nylon compositions and films with small amounts of additive concentrates wherein the compositional combinations attenuate the reduction in the desirable properties of the nylon while incorporating into the nylon the advantageous properties of the additives.
It is another object of the invention to provide multiple layer films including a layer of nylon which incorporates the concentrate, the combination being responsible for attenuating the reduction in the beneficial nylon properties.
It is yet another object to provide packages made with films having the improved nylon compositions.